1, 25(OH) 2 D3 inhibits T cell activation and maturation of dendritic cells and induces tolerogenic dendritic cells. This study was conducted to assess the effect of active vitamin D on co-stimulatory molecules and HLA-DR expression in renal transplant recipients. Renal transplant recipients who were transplanted 6-18 months before the study with stable allograft function, no episode of allograft dysfunction or febrile episode in the last 2 months were administered oral calcitriol for 4 weeks. Expression of HLA-DR, CD28, CD86, CD40 on peripheral blood leukocytes were checked by flowcytometry. Exclusion criteria were induction with Anti-IL-2 receptor blockers or ALG, history of vitamin D administration, change of immunosuppressive drugs or hypercalcemia during study. Table 1. Changes of co-stimulatory and HLA-DR molecules expression before after Percent reduction P value CD 28 2.30±0.74 1.61±0.91 30% 0.004 CD40 3.07±1.51 2.11±1.71 31.2% 0.0001 CD86 2.37±1.00 1.5±0.78 36.7% 0.0001 HLA DR 9.99±3.02 8.31±2.93 16.8% 0.0001 (a): percent of blood leukocytes with expressed marker (counted in at least 15,000 peripheral blood leukocytes by flowcytometry). Mean of serum calcium and creatinine were not changed statistically significant. In conclusion, by this study we have shown changes in co-stimulatory and HLA-DR molecules expression in these renal transplant recipients that may influence allograft survival.